Prevention of marine encrustation on bronze propellers

ABSTRACT

From the time that they are immersed into a marine environment, bronze propellers are prone to attack by marine organisms, such as barnacles, coral and algae, which attach themselves to the bronze metallic surface, creating lumps on the propeller, which adversely affect its balance and cause impedance and vibration of the propeller and its boat in the water. Ant-fouling paints are either too toxic for the marine environment of lack smoothness on the surface. These problems have been overcome by a combination of known steps, namely, polishing the propeller to prepare it for electroplating, cleansing to remove all traces of dirt and grease, electroplating with copper to a depth of at least 0.005″ or 0.15 mm, followed by spraying with a standard solution (5%) of sodium hypochlorite and sodium chloride in a suitable container to form a firmly adhering conversion coating of basic cupric chloride and then sealing for at least twenty-four hours. The preferred procedure for polishing is 60# grit size at 3500 sfm for roughing, followed by 180# grit size at 5500 sfm for finishing using grease as a polishing aid. Electroplating with copper to the minimum depth provides a smoothing effect ranging from 70 to 90 per cent. The container in which the hypochlorite spraying is carried out is preferably kept sealed until just before fitting and launching. A life expectancy of five (5) years can be anticipated with minimal maintenance every time the vessel is slipped.

[0001] This invention relates to a process for the prevention of marine encrustation on components used in a marine environment and in particular, although not exclusively, bronze or copper components, such as bronze propellers. Bronze includes tin bronze, aluminium bronze, silicon—aluminium bronze, nickel—aluminium bronze and manganese bronze.

[0002] A problem with components that are submerged in a marine environment for extended periods is that marine life encrustation can develop on the surface of the component. This is particularly a problem with propellers. In operation, water travels over the propeller blades at high velocity, it is essential for the efficiency of the propeller, that the surface be perfectly smooth, even and true. From the time that they are immersed into a marine environment, bronze propellers are prone to attack by marine organisms, such as barnacles, coral and algae, which attach themselves to the bronze metallic surface, creating lumps on the propeller, which adversely affect its balance and cause impedance and vibration of the propeller and its boat in the water. Various remedies have been tried including anti-fouling paints. One of these paints containing tributyl tin, was so toxic to other economic marine life, such as oysters, that it had to be discontinued. Its successor is so aggressive, that its application to the finely polished surface of the bronze leaves brush marks in the form of grooves, that adversely affect the fine balance and vibration free performance of the propeller. Durability of the anti-fouling paint on the propeller can be as short as 30 days in active marine environments.

[0003] Accordingly, the inventive process seeks to provide protection from marine encrustation for an extended period of time.

[0004] In accordance with the present invention there is provided a process for improving the resistance to marine encrustation of a component used in a marine environment, including the steps of: [treating the component with a hypochlorite or a hypochlorite-containing material or a precursor thereof.]

[0005] providing a component adapted for use in a marine environment, said component having an outer surface containing copper;

[0006] applying a hypochlorite or hypochlorite containing material or a precursor thereof (hereafter hypochlorite material) to said outer surface; and

[0007] allowing reaction of said hypochlorite material with the copper to form an oxide layer at the outer surface;

[0008] whereby the oxide layer formed at the outer surface containing copper resists marine encrustation of said component in a marine environment.

[0009] Also according to the present invention there is provided a process for improving resistance to marine encrustation of a component used in a marine environment including the steps of:

[0010] providing a component adapted for use in a marine environment;

[0011] applying a layer of copper or a copper-containing material to an outer surface of the component;

[0012] applying a hypochlorite or hypochlorite containing material or a precursor thereof (hereafter hypochlorite material) to said outer surface; and

[0013] allowing reaction of said hypochlorite material with the copper to form an oxide layer at the outer surface;

[0014] whereby the oxide layer formed at the outer surface containing copper resists marine encrustation of said component in a marine environment.

[0015] Preferably, the component is made from copper, contains copper or is provided with a surface layer of copper or copper-containing material. More preferably, the process further includes forming a layer of copper or copper-containing material on the surface of the component before treatment by the hypochlorite material, particularly if the surface of the component is not copper or is not provided with a copper or copper-containing surface.

[0016] Preferably, the hypochlorite material is a hypochlorite salt solution or a material capable of forming a hypochlorite salt solution.

[0017] Preferably the oxide layer includes basic cupric chloride.

[0018] [Preferably, a cupric chloride coating is formed on the surface of the component by treating the copper surface or copper surface layer of the component with the hypochlorite salt solution.]

[0019] Preferably, the step of applying a layer of copper or copper-containing material is conducted [component is metallic and the copper layer is formed on the metallic surface] by electroplating. Typically the component [or the surface of the component] is or includes bronze before being electroplated with copper.

[0020] Preferably, the copper surface is exposed to the hypochlorite salt solution for a period of time sufficient for the basic cupric chloride coating to form.

[0021] Preferably, the hypochlorite salt solution is a standard sodium hypochlorite solution containing 5% sodium hypochlorite and 5% sodium chloride.

[0022] Preferably, the hypochlorite salt solution is applied, by [preferably] spraying [sprayed onto] the hypochlorite material onto the component [copper surface or copper layer].

[0023] [Preferably, the step of forming the basic cupric chloride coating is preceded by the formation of cupric oxide.]

[0024] Preferably the outer surface is cleaned prior to electroplating.

[0025] Preferably the surface of the component is polished before it is cleaned.

[0026] Preferably, the polishing procedure includes using a 60# grit size at 3500 sfm for roughing, followed by using a 180# grit size at 5500 sfm for finishing the propeller using grease as a polishing aid.

[0027] Preferably, the surface cleansing includes one or more steps of alkaline cleaning by dipping or electrolytic means, vapour degreasing and solvent cleaning.

[0028] Preferably, the copper layer is electroplated using an electroplating bath which is alkaline. More preferably, the copper electroplating bath is an alkaline cyanide bath. Alternatively, the copper electroplating bath is an alkaline pyrophoshate bath.

[0029] Alternatively, the copper layer is electroplated using an electroplating bath which is acid. Preferably, the copper electroplating bath is an acid sulphate bath. Alternatively, the copper electroplating bath is an acid fluoborate bath.

[0030] In accordance with another aspect of the present invention there is provided a component having increased resistance to marine encrustation in a marine environment, the component being adapted for use in a marine environment, wherein the surface of the component contains copper and has been treated with a hypochlorite or hypochlorite-containing material or precursor thereof, to form [after which the component is provided with] a basic cupric chloride coating, whereby said basic cupric chloride coating resists marine encrustation of said component in a marine environment.

[0031] Preferably, the component is made from copper, contains copper or is provided with a surface layer of copper or copper-containing material on which the basic cupric chloride coating is formed. [More preferably, the copper or copper-containing surface is a layer formed on a metallic surface of the component. Still more preferably, the metallic surface is a bronze surface of the component.] Typically, the component is a bronze propeller.

[0032] Preferably, said surface is in the form of an electroplated layer of copper or copper-containing material [the layer is an electroplated layer].

[0033] Preferably, the basic cupric chloride coating is a cupric oxy-chloride coating. More preferably, the basic cupric chloride coating is a cupric chloro-hypochlorite coating.

[0034] In order to provide a better understanding of the present invention, a embodiment will now be described in detail. The preferred embodiment is described in relation to bronze propellers, however it will be appreciated that the invention is applicable to other components.

[0035] A preferred embodiment of the process for the prevention of marine encrustation on bronze surfaces, in particular those of propellers, is characterised by the following steps:-

[0036] (a) The cast propeller is first polished to the propeller production standard.

[0037] (b) The whole polished propeller is then cleansed, to remove all traces of dirt and grease.

[0038] (c) The cleansed propeller is then electroplated with copper.

[0039] (d) The electroplated propeller is then placed in a suitable container and sprayed with a standard solution of sodium hypochlorite.

[0040] Thereafter, only minimum maintenance is required, when the vessel is slipped periodically, thus providing a substantial reduction in maintenance costs.

[0041] The first step in the preparation is polishing. Bronze propellers and associated structure are typically sand cast and require polishing to remove scale. Reference top this procedure may be found in the article ¹“Mechanical Finishing—Polishing and Buffing”. The recommended procedure is 60# grit size at 3500 sfm for roughing, followed by 180# grit size at 5500 sfm for finishing using grease as a polishing aid.

[0042] The second step in the procedure consists of surface preparation or cleansing to remove all traces of dirt and grease and may consist of one or more of alkaline cleaning by dipping or electrolytic means, vapour degreasing and solvent cleaning in the article ²“Metal Cleaning—Section of Cleaning Process”. This surface preparation is also the subject of standard ASTM B281—“Preparation of Copper and Copper Base Alloys for Electroplating”.

[0043] The third step involves electroplating the cleansed propeller with copper to an operatively effective depth, for example 0.002″ or 0.06 mm. Various salts of copper may be used, but the most common are those of the two alkaline (cyanide and pyrophosphate) baths and the two acid (sulphate and fluoborate) baths. These are variously described in ³“Copper Plating” by Mattie F. McFadden and are the subject of two standards AMS 2418 and MII,-C-14550(Ord). As well as provided an appropriate surface for subsequent processing, electroplating with copper enhances the surface by providing a substantial levelling effect ranging from 70 to 90 per cent for a deposit of 0.005″ in thickness.

[0044] The fourth step in the process consists of placing the electroplated propeller in a suitable container and spraying the electroplated surface with sodium hypochlorite solution. This solution is prepared by chlorination of sodium hydroxide (caustic soda) solution

2NaOH+Cl₂—>NaCl+NaClO+H₂O

[0045] or, alternatively, by electrolysis of a sodium chloride (common salt) solution and reacting the product of the anode (chlorine) with the product of the cathode (sodium hydroxide). Sodium hypochlorite is routinely marketed as a 5% equimolecular solution of sodium chloride and sodium hypochlorite for the disinfection and sterilisation of such places as dairies and milking sheds under various trade names such as Eau de Javelle, Chlorox and Dazzle. The reactions which take place with the copper surface are believed to be

[0046] A first coating of black cupric oxide is formed in reaction (1), which coating is then converted to a blue-green basic cupric chloride in reaction (2). The basic cupric chloride forms a firmly adherent coating, which resists the encroachment of marine organisms. A life expectancy of five years of effective protection against marine growth has been achieved, providing ultimate thrust to manufacturers' standards, together with precise balance and vibration-free performance.

[0047] It will be apparent to persons skilled in the relevant arts that modifications and variations can be made to the described invention without departing from the basic inventive concepts, such as:

[0048] (i) the surface treated to form the basic cupric chloride coating may already be copper and therefore copper electroplating may not be necessary;

[0049] (ii) a component may not necessarily be bronze, so long as it is electroplated with copper on which is formed the basic cupric chloride coating;

[0050] (iii) a standard solution of sodium hypochlorite may be substituted with other hypochlorite salt solutions;

[0051] (iv) the hypochlorite salt solution may be applied in various other ways to the copper surface than spraying, such as painting or dipping; and,

[0052] (v) the time of exposure to the hypochlorite solution may vary, provided that it is long enough for an operatively effective basic cupric chloride coating to form.

[0053] All such modifications and variations as would be apparent to a skilled addressee are intended to be included with the scope of the present invention, the nature of which is to be determined from the foregoing description and appended claims.

BIBLIOGRAPHY

[0054]^(1,2,3) Metals Handbook, 8th Edit., Vol. 2—“Heat Treating, Cleaning and Finishing”. (ASM, Metals Park, Ohio 1964). 

The claims defining the invention are as follow:-
 1. A process for the prevention of marine encrustation on bronze surfaces, in particular those of propellers, to provide protection from marine activity for an extended period of time, characterised by the following steps:- (a) the cast propeller is first polished to the propeller production standard; (b) the whole polished propeller is then cleansed, to remove all traces of dirt and grease; (c) the cleansed propeller is then electroplated with copper to a depth of at least 0.005″ or 0.15 mm; (d) the electroplated propelle is then placed in a suitable container and sprayed with a standard solution of sodium hypochlorite; (e) the container is then sealed for at least twenty-four hours.
 2. A process according to claim 1, wherein the polishing procedure is 60# grit size at 3500 sfm for roughing, followed by 180# grit size at 5500 sfm for finishing using grease as a polishing aid.
 3. A process according to claim 1, wherein the surface cleansing consists of one or more of alkaline cleaning by dipping or electrolytic means, vapour degreasing and solvent cleaning.
 4. A process according to claim 1, wherein the copper electroplating baths are alkaline.
 5. A process according to claim 4, wherein the copper electroplating bath is an alkaline cyanide bath.
 6. A process according to claim 4, wherein the copper electroplating bath is an alkaline pyrophosphate bath.
 7. A process according to claim 1, wherein the copper electroplating baths are acid.
 8. A process according to claim 7, wherein the copper electroplating bath is an acid sulphate bath.
 9. A process according to claim 7, wherein the copper electroplating bath is an acid fluoborate bath.
 10. A process according to claim 1, wherein the container is kept sealed until just prior to launching. 